The present invention relates to an analog signal level detector.
The term level is here understood in its widest sense, because the applications envisaged for this detector essentially relate to knowing whether the expected signal is present or absent. The signal is expected, i.e. the range of its spectrum of frequencies is approximately known, together with its approximate amplitude, even if the range of possibilities is relatively wide. For example, a modem (modulator-demodulator) connected to a telephone line would recognise the presence of a call signal constituted by the carrier of known frequency.
Other similar or even relatively different applications can be envisaged. A voice activity detector in a speech analyzer could be useful for only initiating analysis operations in the presence of a signal manifesting the established existence of a voice activity with an adequate level.
Consequently the level detector in question can be a mean or average level detector of the rectified signal, without the mean or average word having to be taken in the strict mathematical sense. The object is to supply a signal recognizing the presence of an input signal if said mean value exceeds a predetermined threshold.
Such a detection requires a low-pass filtering, i.e. a certain signal integration with a time constant well above the variation period of the signal received. Without this there would be a risk of alternately detecting the presence and absence of the signal and each peak and each trough of the rectified alternating signal.
Thus, it is on the one hand necessary to rectify (preferably full-wave rectification) the signal to be detected and on the other make it undergo a certain integration.
Unfortunately, the integration of an analog signal with a relatively long time constant (e.g. 200 milliseconds for a 50 Hz signal) requires capacitors having relatively high values, which makes it difficult or even impossible to produce the signal detector in the form of an integrated circuit. For example for a modem, it is not only desirable for the detector to be in integrated form, but it is also desirable for it to form part of the same semiconductor chip as the actual modem.